How Many Pixels are Enough?

TV makers and retailers are revving up their ultra-high definition (UHD) awareness programs. Thus, as night follows day, critics are intensifying their argument that UHD (or 4Kx2K) television cannot be distinguished from today’s full-high-definition or FHD (or 2Kx1K) television at the distances American and European viewers typically watch TVs in their rooms.

This argument is not new. Those of you with a few gray hairs will remember it being made when HDTV began to replace standard-definition (SD) CRT-based and plasma-based TVs. The argument, which I’ve used myself, is based on the acuity of the human visual system. As measured by the famous Snellen Eye Chart and its successors in your ophthalmologist’s office, the threshold for recognizing letters is 5 minutes of arc for people with 20/20 vision. The letters on the eye chart can be represented with as few as 5 pixels vertically, so this is the equivalent of 60 pixels per degree (ppd). For some digital context, the 4-inch display on Apple’s iPhone 5 has 308 pixels per inch (ppi), which is 54 ppd at a 10-inch viewing angle. The higher-resolution 5-inch display on Samsung’s Galaxy 4 has 77 ppd at the same viewing angle.

For TV, a 56-inch FHD set viewed at the SMPTE standard distance of 90 inches (for this size of display) has 62 ppd. A 4K set viewed at the same distance has 124 ppd. Based on this information, the digital Luddites would seem to be correct. The FHD set’s pixel density exceeds what a person with normal vision can see at a viewing distance of 7 1/2 feet.

But is this the entire story? It isn’t, and for two reasons that become rather obvious once you think about them. First, the images on the Snellen Eye Chart don’t move; and, second, the moving images on a TV set — which started out as continuous images in real life — are being presented on the TV screen by a fixed matrix of pixels. It’s a simplification (because the real-world image was originally digitized by a camera), but think of the original moving image overlaid by the TV set’s fixed pixel matrix. We perceive the original moving image, but what we actually see are flickers of light from the fixed pixels.

Now, even if the pixel density is so great that we can’t resolve the individual pixels, does the “movement” of the image relative to the fixed pixel matrix create artifacts that we can see? The answer – as presented in a recent, rather technical white paper by Pixelworks entitled “High Resolution Displays and Moving Images” – is yes.

The Pixelworks white paper – the first of three on the subject – collects existing information, combines it with new research on human subjects performed by the company, and makes a persuasive argument that our “dynamic acuity” when watching a fixed-pixel display is considerable greater than the passive acuity measured by the Snellen Chart, and that a greater pixel density is required before “we no longer notice any artifacts caused by the discrete nature of the pixels”.
I recommend the white paper if you’re interested in the details of the argument. For now, I will cut to two statements from the paper’s conclusion.

1. “When the sampling-phase effects are taken into account, the perceivable pixel density limit increases from 60 ppd to nearly 90 ppd”.

2. “Common video processing techniques can cause the fixed- pixel structure of the display to be visible regardless of the display resolution. Fortunately, alternative approaches are available that can greatly reduce the effect of fixed- pixel sampling, as will be discussed in the next paper in this series”.

In other words, you can see the difference between FHD and UHD, even on moderately sized TV screens at normal viewing distances – and you can now go and buy a new UHD-TV with a clear conscience. – Ken Werner

Ken Werner is Principal of Nutmeg Consultants, specializing in the display industry, manufacturing, technology, and applications, including mobile devices and television. He consults for attorneys, investment analysts, and display-related companies. You can reach him at [email protected].

Related Posts